In the critical care department of a hospital, there are patients that are seriously ill and of whom the cardiovascular function needs continuous attention. Often these patients also are mechanically ventilated, either continuously or during surgery. The monitoring of the cardiovascular status of these patients is done by continuous measurement of the blood pressure and the heart rate. These parameters, however, are often not accurate enough to follow the cardiovascular function in time and to make a good statement about the significance of the changes.
Therefore other methods have been developed, based on mandatory mechanical ventilation. One of the known methods is the “respiratory systolic variation test” (RSVT). The RSVT is a measure of the slope of the decrease in the systolic arterial blood pressure in response to a standardized maneuver consisting of a series of successive incremental pressure-controlled breaths. Such a method is described in U.S. Pat. No. 5,769,082 by Perel. The procedure as proposed in U.S. Pat. No. 5,769,082 can be described as follows. The patient is ventilated by a mechanical ventilator in a mandatory ventilation mode, such as Volume Controlled Mandatory Ventilation (VCCMV) or Pressure Controlled Mandatory Ventilation (PC-CMV). In order to measure the cardiovascular status a respiratory maneuver is made. A sequence of consecutive tidal volumes of increasing magnitude is applied to the ventilated patient. These varying pressures cause a reaction of the cardiovascular system.
U.S. Pat. No. 5,769,082 further discloses a short apnea period before the series of tidal volumes with varying magnitude is applied. This may be done to have a more significant effect of the variation in pressure. FIG. 1A illustrates an example of measured blood pressure values in mm Hg, and FIG. 1B illustrates an example the airway pressure in cm H20, both as a function of time. T1 to T2 indicates the apnea period. From T2 to T3, the airway pressure of successive tides is increased; as a consequence, the higher blood pressure increases whereas the lower blood pressure decreases. The slope of a line connecting the lower blood pressures is determined in the RSVT.